Add WebInputEvent::GestureShowPress, PlatformEvent::GestureShowPress.

Working towards having consistent definitions of tap down and show press
gestures between blink and chromium. This is the first of 4 patches (see the bug
for details).

In this patch we only listen for show press events, but convert tap down events
to show press events as they are received. This results in no change of
functionality, but will allow chromium to start passing show press events to
blink.

BUG=302752 TEST=Existing layout tests, manual testing with chromium passing
either tap down or show press events.

Review URL: https://codereview.chromium.org/26030002

git-svn-id: svn://svn.chromium.org/blink/trunk@159060 bbb929c8-8fbe-4397-9dbb-9b2b20218538